1. Related Applications
The present application is related in subject matter to the following U.S. patent applications filed simultaneously herewith: Ser. No. 341,726 filed in the names of Barry N. Stone, Richard C. Rice, John E. Lorbiecki and Stanley Berstein and entitled APPARATUS AND METHOD FOR CONTROLLING FILM DENSITY FOR AN APPARATUS FOR EXPOSING PHOTOGRAPHIC FILM WITH IMAGE DATA; and Ser. No. 341,730 filed in the names of Richard C. Rice, Barry N. Stone and Lynn J. Orendcrff and entitled SYSTEM AND METHOD FOR CONTROLLING EXPOSURE FORMAT FOR AN APPARATUS FOR EXPOSING PHOTOGRAPHIC FILM WITH IMAGE DATA.
2. Field of the Invention
The present invention generally relates to apparatuses for exposing photographic film with image data and, in particular, to calibration of the light source for such apparatuses.
3. Statement of the Prior Art
Photographic film has become the accepted archival medium for medical imaging because of its high analog resolution and because of its long time use with x-rays. Conversely, modern medical imaging techniques, such as computerized tomography, digital radiographic imaging, ultrasound and magnetic resonance imaging, have developed around the use of digital imaging techniques where an image is made up of a great many pixels each of which has an electronically represented brightness. In order to adapt these new techniques to the accepted archival medium, apparatuses have been developed for creating a hard copy by "printing" the electronic image data onto photographic film. These apparatuses are generally referred to as hard-copy cameras.
Thus far, the two primary approaches have used lasers and cathode ray tubes as light sources to expose the film by illuminating each pixel to the proper intensity. Cathode ray tubes at least have experienced some problems in controlling the exposure or writing intensity because of the inherent instability of the cathode ray tube intensity. This varies readily with temperature and causes calibration over a range of intensities to be practically impossible. Thus, it is desirable to have a conveniently implemented calibration procedure which may be regularly used to maintain proper exposure intensity.
One attempt at controlling the various factors involved is described in U.S. Pat. No. 4,700,058. This patent describes a scheme for calibrating exposure intensity against a pair of predetermined reference levels. This is done both before a measurement of film density and during the exposure process to retain the validity of the density measurements as well as maintain the intensity stability. Unfortunately, this approach is limited to the number of reference voltages set up for comparison, and the reference voltages have no relation to the intensities determined to produce desired film densities. Therefore, it is possible that as "calibrated", the actual light intensity levels differ significantly from the light intensity levels corresponding to the densities at which the film is measured. In other words, the film density may be measured using one set of exposure intensity levels while the exposure intensity is calibrated at a different level. This condition can introduce gray scale inaccuracies due to the nonlinear responses of the light source, the film and the developing chemicals.